Project Summary Prostate cancer is the most common malignancy among men in western countries. Prostate tumors initially respond well to androgen ablation or anti-androgen therapy, but eventually enter an androgen-independent stage with no effective therapy. Clearly, new therapeutic approaches are needed, and this will require a better understanding of the signaling events that control prostate tumorigenesis. Our laboratory has identified a new tumor suppressor, Par-4, which has pro-apoptotic activity and plays a role in human prostate tumorigenesis. Our preliminary results have demonstrated that Par-4 is lost in 60% of human prostate tumors, and that it binds and inhibits PKC, consequently reducing NF-B and Akt activation, and increasing cell death. Interestingly, tissue microarray analysis of human prostate carcinomas revealed a correlation between PKC expression and increased Ki67 labeling indexes. Moreover, cancer gene-expression profiles comparing PKC levels in different stages of human prostate neoplasias showed that PKC expression was strongly correlated with a high degree of tumor aggressiveness. Therefore, the Par-4/PKC complex appears to be a relevant candidate mediator of prostate tumorigenesis. In preliminary studies, we found that Par-4-/- mice developed benign hyperplasia and prostate intraepithelial neoplasias (PIN) that could progress to prostate adenocarcinomas when combined with PTEN heterozygous deletion. Therefore, Par-4 emerges as a novel tumor suppressor through its ability to impinge on two critical signaling pathways, NF-B and Akt, likely through PKC. The long-term goal of the studies proposed here is to unravel the signaling cascades involved in prostate cancer initiation and progression. This work will test the hypothesis that the loss of Par-4 in combination with PTEN haploinsufficiency triggers invasive prostate adenocarcinoma, and will determine the cellular and molecular signaling pathways that control that process. Advances in the understanding of these phenomena may uncover new perspectives on prostate carcinogenesis, and provide novel targets for prostate cancer prevention, diagnosis, and therapy. Therefore, in this proposal we will 1) test the hypothesis that Par-4 deficiency in combination with PTEN heterozygosity leads to the generation of invasive prostate cancer; and 2) determine the Par-4-mediated cellular and molecular mechanisms that are involved in prostate cancer progression in the context of PTEN haploinsufficiency. This work will increase our understanding of the mechanisms involved in the regulation of prostate carcinogenesis, and in the long term will provide the knowledge necessary for the development of novel, more specific, and thus less toxic, therapies for the treatment of prostate cancer.